Printed wiring boards, circuit boards and other elements now have smaller wettable surfaces to be solder coated and joined. Whereas in the past flux was used to remove oxides from these surfaces, it has now been found that if soldering occurs within an inert gas or a shield gas atmosphere which excludes oxygen, then flux may not be necessary. In the present application, the term "inert" as applied to gas includes nitrogen. Furthermore dross and other oxides do not form on the surface of solder in an inert atmosphere. In the past dross and oxide particles have caused skips, bridges, or other defects to occur in soldered joints.
In U.S. Pat. No. 5,048,746 is disclosed a tunnel for fluxless soldering. In U.S. Pat. No. 5,044,542 is shown shield gas wave soldering. In both cases solder is applied to an element in a shield gas environment. The shield gas may be inert gas or a reducing gas which excludes oxygen. The benefits of soldering in an atmosphere which excludes oxygen are known.
Tardoskegyi in U.S. Pat. No. 3,705,457 and Elliott et al in U.S. patent application Ser. No. 549,603 both disclose the use of a gas knife jet, but only for applying a gas jet on one side of a solder wave, generally the downstream side.